Packet RadioIntroduction to High Speed Packet(Is FM a dead end?) Most people think of packet as an inherntly slow mode, 1200 Baud VHF or 300 Baud HF. But packet can be any speed you want, limited by bandwidth and also the symbols per clock of the modulation method. Basic FSK (FM or HF Frequency Shift Keyed packet) only has 1 symbol per "clock". CW (dit, dah or space are the symbols) has LESS than one symbol per clock, if the dit is shortest interval and a word space the longest. QPSK has four possible states, so two symbols can be transmitted at once. COFDM (used in Digital Terrestrial TV) and Q15X25 use multiple QPSK, QAM or PSK carriers, so the symbol rate is multiplied by the number of carriers. Of course the overall rate may be the same as each carrier uses a slower modulation. Many people think that because sound card based Packet such as MixW, AWGPE, Linux AX.25 or Flexnet32 can do 9,600 or even 19,200 packet and many modern rigs have "Data" ports (actually unfiltered audio connections), that this is the solution to 1200 baud crawls. Actually this is a myth. Why?
A Tx/Rx relay can switch in 8ms to 50ms depending on type. About 12ms might be typical, yet FM only rigs may need 150ms (newer) to 800ms (older) to stabilise on transmit. This is because often whole sections are unpowered and powered (with charge up time of decoupling capacitors) on keyup. Any rig designed to allow CW break-in or CW semi-breaking on SSB etc, can often key up much faster. Even an old FT101ZD hybrid on HF may achieve less than 50ms Tx delay on HF data transfer (300 baud packet, 1200 Pactor II, 2,500bps Q15X25 or MT63 etc). An analog phone line has only 3.5Khz bandwidth. Leaving aside how ISDN (144K full duplex) or xDSL (up to 10Mbps) work, analog modems can run at up to 33Kbps. The 56k is not a true analog modem mode, it only works to a terminal unit in a Digital Exchange, a kind of quasi U.S. ISDN mode. So how does 33kbps fit down a phone line? Note the bps rather than baud. Actually it may be around 9.6K baud! Baud is symbols per second. Which for 300 Baud and 1200 Baud may indeed be bits per second, (bps). but higher modem speeds do not use frequency shift keying (FSK, as 300baud and 1200baud packet use), which is 1200Hz and 2200Hz to represent the "0" and "1". In fact standard HF and VHF FSK packet is EXACTLY old 300 baud and 1200 baud telephone modem tones! The next development was Phase Shift Keying (PSK) of a single frequency carrier. This allows a higher symbol or bit rate in the same 3.5KHz bandwidth than FSK. PSK31 is a 31 bps PSK modulation of a single carrier. Q15X25 is fifteen equally spaced carriers each PSK modulated at a 83 bps rate. But how can we fit more? Well we could have four phases instead of two (0 and 180). This means that two bits can be sent at once (fig 1) Example only
fig 1 This is called QPSK, Qudrature Phase Shift Keying. It and PSK are regarded as a special cases of QAM, Quadrature Amplitude Modulation (pronounced quam). In "QAM 16" there are 16 combinations of phase and also amplitude of the carrier. This means that FOUR bits can be sent simultanously. This means QAM 16 of " 1000 baud" or symbol rate of 1000 symbols per second is actually 4000bps. A little reflection will result in the realisation that while using an SSB rig is possible, with an FM rig, PSK, QPSK and any form of QAM is impossible. Only the frequency of an FM rig can be modulated. While an SSB rig is possible for PSK and QPSK and even OFDM PSK (Multiple equally spaced carriers, all always on = Orthagonal Frequency Division Multiplexing), it has limitations for higher QAM numbers or achieving the best S/N performance for PSK, QPSK or OFDM. So techincally, for higher speed, an FM rig is a dead end. An SSB rig is possible, but often do not have a suitable IF response. To achieve 33K bps a dedicated Tx/Rx is required. However a very good linear phase response SSB receiver and transmitter might actually work with a slightly modified 33kbps telephone modem. No FM only rig ever would. Using a 25Khz 2m or 70cm channel a maximum datarate might be possible of over 150k. This would only work on Cable or Geosyncronous Satellite. Normal Amsat would have too much doppler shift and terrestrial transmission too much mulitpath. On 2m and 70cm, though 38.4K is common in some countries. To achieve higher rates an OFDM (Such as Q15X25 on HF) or COFDM (Coded Orthagonal Frequency Division Multiplex) is needed. This is because each of the 15 to 2500 carriers runs at a lower bit rate. Also if "Forward Error Coding" is used, then if a few carriers are lost due to QRM, then the data is not lost. Why up to 2500 carriers? Well at very low symbol rates, DSP (Digital Signal Processing) can "hear" the different multipath signals arrive at different times, on the single channel reciever and "slide" the signal in time (it would be in internal memory locations as a digital numbers repesenting the amount of signal at each instant of sample, perhaps up to 100 Million per second) to "align" them and add them together. But since noise is random, not only have you solved the multipath problem but actually improved S/N performance by maybe 6dB! This is NOT state of the art. Commercial receivers are under $100 and it is ten year or more old technology. Forget 9600 baud FM. Think 28K and higher.
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